As shown in FIG. 22, a structure of a conventional loudspeaker includes a magnetic circuit 1, voice coil member 4, diaphragm 5 and frame 7. The voice coil member 4, which has movable coil 3, is disposed in a magnetic gap 2 of the magnetic circuit 1. An inner peripheral part of the diaphragm 5 is linked with the voice coil member 4 outside the magnetic gap 2. An outer peripheral part of the diaphragm 5 is linked with the frame 7 via an edge 6. An electric signal, which is supplied from an audio amplifier and the like, is input to the coil 3 of the voice coil member 4, and the voice coil member 4 is then excited. As a result, force is transmitted to the diaphragm 5, and then the diaphragm 5 vibrates air, thereby changing the electric signal into sound.
As shown in FIG. 22, an inner peripheral part of a damper 8 is fixed between the coil 3 of the voice coil member 4 and a fixed point of the inner peripheral part of the diaphragm 5. An outer peripheral part of the damper 8 is fixed at the frame 7. A damper 8 and an edge 6 form a suspension, and prevent the voice coil member 4 from rolling during operation. As shown in FIG. 22, the damper 8 is formed of a plurality of wave shapes for reducing a mechanical load of the voice coil member 4.
In this structure mentioned above, in working of the voice coil member 4 toward the magnetic circuit 1 and working of the voice coil member 4 toward an opposite side of the magnetic circuit 1, non-linearity and asymmetry of a mechanical load of the damper 8 becomes large. As a result, large harmonic distortion occurs, and power linearity deteriorates. FIG. 23 shows an amplitude of the diaphragm 5 vs. an input electric power of the loudspeaker, namely power linearity of a conventional loudspeaker having the damper 8. Curve A shows an amplitude characteristic of the diaphragm 5 toward the magnetic circuit 1, and curve B shows an amplitude characteristic of the diaphragm 5 toward an opposite side of the magnetic circuit 1. FIG. 24 shows a harmonic distortion characteristic of the conventional loudspeaker having the damper 8, where curve C shows an output sound pressure of the loudspeaker, curve D shows the second harmonic distortion characteristic and curve E shows the third harmonic distortion characteristic. As mentioned above, the damper 8 is formed of a plurality of wave shapes for reducing a mechanical load of the voice coil member 4, and the damper 8 and the edge 6 form a suspension. As a result, it is difficult to improve non-linearity and asymmetry in the conventional loudspeaker, so that harmonic distortion is not reduced.